The present invention relates generally to oscillators, crystal oscillators, and more particularly to stabilizing circuits for crystal oscillators to ensure operation over temperature changes.
Requirements for smaller, lower power, and less expensive electronics have lead to more integration to achieve these goals. Many circuit functions are being combined onto a single silicon die. One of the most inexpensive of integration processes is bulk CMOS well known in the art. There are many advantages to using CMOS including low power, high-density integration, and low cost. One problem with the CMOS process is a wide variation of transistor gain over a temperature range of xe2x88x9255xc2x0 C. to +105xc2x0 C. typically required for military and some commercial applications.
Providing a high-stability reference frequency from a frequency standard is an important part of many communication and navigation systems operating in environments subject to substantial variations in temperature. Typically, the reference frequency is provided using a transistor-driven oscillator circuit having a crystal to establish a selected operating frequency. For example, feedback oscillator circuits, such as the Colpitts, Pierce or Hartley types, operate by returning a portion of the output signal to the input to sustain oscillation by positive feedback. Achieving a reference frequency that is highly stable in such a transistor-driven oscillator circuit typically requires use of a temperature-controlled quartz-crystal oscillator.
When using CMOS transistors to develop high-stability frequency standards, the variation of transistor gain with temperature presents problems for many of the functions of the frequency standard, especially the oscillator circuit itself. Quartz crystals used in high-stability frequency standards in many applications have an optimum operating current. If the crystal drive current is too high, the crystal will exhibit discontinuities in the crystal response curve (activity dips). The crystal may operate in spurious modes if the drive current is too high. If the crystal drive current is too low the crystal oscillator will not start operating over all temperatures. The transistor gain variation in an integrated CMOS process can vary in excess of 2 to 1 over temperature. This variation makes it very difficult to design an oscillator circuit that has good start up over temperature and at the same time does not cause activity dips in the crystal.
U.S. Pat. No. 5,341,112 entitled TEMPERATURE STABLE OSCILLATOR CIRCUIT APPARATUS by Leo J. Haman and assigned to the assignee of the present invention discloses a crystal oscillator with a bias circuit that controls the bias by controlling the emitter voltage of a bipolar transistor. The bipolar-transistor crystal oscillator circuit is a modified version of a conventional transistor crystal oscillator, such as a Hartley, Pierce or Colpitts-type circuit. The bias circuit includes a current source providing a reference current through a Schottky diode and a pair of bipolar transistors. The bipolar transistor crystal oscillator includes a bias input coupled to the bias circuit. The bipolar-transistor crystal oscillator provides a second current through a second bipolar transistor. The second current tracks the reference current so that the output of the bipolar transistor-driven oscillator circuit is substantially constant over variations in ambient temperature. The invention disclosed in the referenced patent is suitable for use with a bipolar transistor oscillator.
What is needed is a CMOS transistor crystal oscillator circuit that offers stable gain characteristics over temperature for use in reliable high-stability frequency standards.
A temperature stabilized CMOS oscillator circuit with stabilized gain over temperature is disclosed. The temperature stabilized oscillator comprises a crystal oscillator with a CMOS transistor. A bias circuit is connected to the crystal oscillator as a current source to provide a supply current to the CMOS oscillator transistor and to vary the supply current over temperature to provide the stabilized gain. The bias circuit includes a primary current mirror connected to the crystal oscillator to provide the supply current to the CMOS oscillator transistor. A secondary current mirror is connected to the primary current mirror to divert current from the primary current mirror to vary the supply current over temperature. The CMOS crystal oscillator may be a Pierce, Colpitts, Clapp, or Hartley oscillator known in the art. The primary current mirror has a primary pair of CMOS transistors connected as a current source. A resistor with a low positive temperature coefficient is connected to the primary pair of transistors to set a primary current mirror current that sets the supply current. The secondary current mirror has a secondary pair of CMOS transistors connected as a current source. A resistor with a high positive temperature coefficient is connected to the secondary pair of transistors to set a secondary current mirror current that diverts current from the primary current mirror current over temperature.
It is an object of the present invention to provide a CMOS transistor crystal oscillator circuit that offers stable gain characteristics over temperature for use in reliable high-stability frequency standards.
It is an advantage of the present invention to utilize two current mirrors to provide a temperature compensating current to an oscillator transistor.
It is a feature of the present invention to be able to incorporate the temperature stabilized CMOS oscillator circuit in a low power, low cost, and high integration density CMOS integrated circuit process.